In recent years, in the field of the fuel tank for automobiles, the formation of a product from a resin has been actively promoted for the purpose of weight reduction and energy saving. As a resin material, from the viewpoints of inexpensiveness, high strength, good weather resistance, good chemical resistance, recycling ability, and the like, a polyethylene has been generally used as a main material. Further, the resin-made fuel tank has been produced by a large-sized blow molding as a mainstream.
However, the large-sized blow molding has problems that the molding machine is specialized, the productivity decreases due to the generation of waste materials such as burr, wall thickness distribution of the molded article becomes uneven, fuel permeability from a blow-molded pinch part, and the like.
In order to solve these problems, there has been proposed plastic fuel tanks and the like produced by molding methods other than the blow molding, for example, an injection molding method (e.g., see Patent Documents 1 to 4).
For example, Patent Document 1 discloses a fuel tank for automobiles, which tank is composed of a polyethylene having specific density, dispersion index, and melt flow index.
Patent Document 2 proposes a polyethylene-based resin for an injection-molded plastic fuel tank, wherein (1) a density is 0.940 to 0.970 g/cm3, (2) a high load melt flow rate (HLMFR) is 6 g/10 minutes or more, (3) a shear viscosity at 230° C. and a shear rate of 243 sec−1 is 10,000 poise or less, (4) a Charpy impact strength at −40° C. is 5 kJ/m2 or more, and (5) a rupture time in a full notch creep test (measured at 80° C., 6 MPa) is 80 hours or more, as an injection-molded plastic fuel tank excellent in a balance among injection moldability, durability, and impact resistance.
Further, Patent Document 3 proposes a polyethylene-based resin molding material for a container, which material is a composition containing an ethylene-based polymer component (A) having a high load melt flow rate (HLMFR) of 0.1 to 1.0 g/10 minutes and a density of 0.910 to 0.930 g/cm3 in an amount of 20% by weight or more and less than 30% by weight and an ethylene-based polymer component (B) having a melt flow rate (MFR) of 150 g/10 minutes or more and less than 400 g/10 minutes and a density of 0.960 g/cm3 or more in an amount of more than 70% by weight and 80% by weight or less and which material satisfies characteristic (1) (MFR being 0.4 g/10 minutes or more and less than 2.0 g/10 minutes and HLMFR being 70 g/10 minutes or more and less than 180 g/10 minutes, and HLMFR/MFR being 100 to 200) and characteristic (2) (density being 0.953 g/cm3 or more and less than 0.965 g/cm3), as a polyethylene-based resin molding material for a container, the use application being different from the fuel tank for automobiles.
Moreover, Patent Document 4 proposes a high density polyethylene composition comprising a first component, the first component being a high-molecular-weight ethylene-α-olefin copolymer having a density ranging from 0.915 to 0.940 g/cm3 and a melt index (I21.6) ranging from 0.5 to 10 g/10 minutes, and a second component, the second component being a low-molecular-weight ethylene-based polymer having a density ranging from 0.965 to 0.980 g/cm3 and a melt index (12) ranging from 50 to 1500 g/10 minutes, wherein the high density polyethylene composition has a melt index (I2) of at least 1 g/10 minutes, a density ranging from 0.950 to 0.960 g/cm3, and a g′ of 1 or more.